Walk to the park at the corner, have your child shout a few times, they’ll all pop their heads out of the door at the sound of others playing, desperate to do anything other than sit inside with their parents.
Second, if the park isn't within shouting distance of the necessary number of kids, half of the ones within shouting distance are at parent-scheduled activities, and half of the remainder aren't allowed to brave street crossings because of the six-foot-tall hoods and inattentive drivers of passing ChildMurker1050s, no amount of park yelling will be sufficient.